This invention relates to ultrasonic inspection of materials bounding circular bores and, more particularly, to boresonic inspection apparatus having bore centering features and transducer carrying devices which are insertable in the bores.
Steam turbine rotors are often circularly bored for purposes which include removal of inclusions and impurities inherently induced by metal casting processes and to reduce the time necessary for heating the rotor to a substantially constant temperature across its radial dimension when steam is introduced on the radial outside thereof. To insure high reliability and continuous turbine operation inspection procedures are periodically scheduled for the purposes of detecting damage to components and affecting repairs to prevent damage resulting from failure of those components. Inspection of the turbine rotor for cracks, defects, and other imperfections has become an important part of such inspection procedures. Ultrasonic inspection from inside the rotor bore presently constitutes a popular inspection process due to its precision, ease of implementation, and speed.
Heretofore, ultrasonic transducers of various types were affixed to a support device which was inserted in the rotor bores and radially expanded to engage the transducers with the bore walls. The support devices provided a self-centering action by simultaneously radially displacing three circumferentially equally spaced transducers into contact with the bore wall. The three transducers were circumferentially rotated at selected axial positions about the bore's longitudinal centerline during engagement with the bore wall. Such inspection procedures often included ultrasonically inspecting each of the selected axial locations with ultrasonic transducers having specialized capabilities such as detection of radial cracks, longitudinal cracks, and circumferential cracks. Since the ultrasonic transducers, by their nature, necessarily contacted the bore wall, the transducers were radially engaged with the bore wall by the self-centering action. The transducers thus provided the means for inspection and constituted the support device's bore wall engaging member. It has been found that proper self-centering action obtains when a force level of 50-100 pounds is exerted on the bore wall by each of the three bore wall engaging transducers. It has also been found that a substantially lesser engagement force of between 5 and 10 pounds on each transducer is optimum from an ultrasonic standpoint to minimize the error in back wall ultrasonic reflections. Exertion of such centering forces directly on the transducer sometimes squeezed sonic coupling material (often an oil film) away from the transducer lens so as to cause an inaccurate or incomplete indication of the inspected material content. Furthermore, squeezing the ultrasonic coupling material away from the transducer lens and simultaneously rotating the transducers sometimes damaged the transducers resulting in lost time and expense in repairing or replacing the transducers.